Impedance circuits may comprise a tunable capacitor so that the characteristics of the impedance circuit can be controlled by a control signal.
U.S. Publication 2009/0134960 A1 describes several varactor circuits. U.S. Publication 2008/0122553 A1 refers to an adaptive impedance matching module comprising three tunable capacitors. U.S. Publication 2007/0247237 A1 is related to a switched capacitor array, wherein a series circuit of two varactors is coupled in parallel to the switched capacitor array.